Atomic clocks are essential to today's increasingly digitized world and are being provided in miniaturized forms as chip scale atomic clocks (CSACs). Atomic clocks use the frequency of the electronic transition of an alkali metal vapor as a frequency reference. Alkali metal gases, such as cesium, rubidium or other atoms with a single electron in the outer shell, undergo optical transitions at very high discrete frequencies in the hundreds of GHz, e.g., optical wavelengths of around 800-900 nm. Atomic clocks optically interrogate a vaporized alkali gas over a bandwidth that includes the transition frequency; several techniques including optical absorption, electrically induced optical transparency, and coherent population trapping are used to define the frequency of the optical transition. These atomic clocks have multiple issues, e.g., a requirement for thermal stability of both the laser optical source and the electronic transition vapor cell (often requiring heating circuitry), complex circuitry and magnetic shielding. Accordingly, these clocks suffer from relatively high power consumption, as well as additional costs and space for the required circuitry and optical components.
Work is now being done to develop millimeter wave CSACs that use the rotational transition of a dipolar molecular vapor, e.g., H2O, as millimeter wave CSACs may allow the use of less complex circuitry than atomic clocks. A major challenge in millimeter wave CSACs is to maintain a very low pressure inside a hermetic cavity containing the dipolar molecule vapor for an extended period of time, e.g., three to five years. The quality factor of the transition to be measured quickly degrades when the pressure inside the cavity increases. Desirable leakage rates are not achievable using current wafer bonding techniques.